Usage of an indented bullet does not imply the statement is a response.

Given Euron's timely entrance soon after Balon's fatal accident, and Euron's lack of morals, it could be that Euron somehow had his brother Balon murdered.

One might argue King Stannis Baratheon murdered Balon Greyjoy along with Robb Stark and Joffrey Baratheon when he burned leeches filled with king's blood in an attempt to magically ensure their deaths. Robb was soon murdered by the Freys and Joffrey by Littlefinger and the Queen of Thorns. Assuming this ritual had its intended effect, it obviously doesn't directly or inexplicably kill its victims but perhaps provides a twist of fate that will ensure their death by some other means. It would then seem out of place for Balon to suddenly die by a fluke accident. It is more likely that he was murdered, just like the other two targets of the spell.

Supporting Evidence

"I dreamt of a man without a face, waiting on a bridge that swayed and swung. On his shoulder perched a drowned crow with seaweed hanging from his wings."

”

This implies that Balon was killed by a faceless man - the man without a face - hired by Euron - the drowned crow (as Euron is nicknamed the Crow's-Eye).

Euron Greyjoy has knowledge of the eastern cities and it's possible he hired a Faceless Man to kill his brother for him. The Faceless Men specialize in murders that look like accidents.

Euron should have been months away from Pyke, but shows up very shortly after the death of Balon, the very brother and king who exiled him. This suggests he was expecting his brother would die.

Questions

If Euron hired a Faceless Man, how did he pay for it? The cost is commensurate with the target, and Balon Greyjoy is a King, more or less, so this would be very expensive, possibly costing years of plunder. One theory suggests that Euron may have paid the Faceless Man with his dragon egg and not tossed the egg into the sea as he claimed.